Autonomous underwater vehicles (AUVs) are inherently energy-limited because they must carry their own batteries or other power source. They are also typically ballasted to be positively buoyant. While positive buoyancy is convenient for floating on the surface and obtaining a GPS fix, or as a fail-safe in the event of a system failure, positive buoyance requires an AUV to expend more energy during a dive, or to maintain a constant depth. AUVs typically are pitched downward in order to descend to a desired depth. While pitched, the body of the vehicle acts like an upside-down airplane wing, providing downward “lift” at the expense of increased drag. This drag must be overcome with additional propulsive force, which in turn requires more power, reducing the length of a mission the AUV can perform before the batteries are depleted. A neutrally buoyant AUV with the same battery capacity and components would have an extended mission length as opposed to a positively buoyant AUV.
One current solution to the positive-buoyancy problem is to incorporate active ballast systems into platforms specially designed for long endurance missions for maximum propulsion efficiency. However, active ballast systems are costly in both power consumption and additional weight and space incorporated into the vehicle. Therefore, they are not widely used in AUVs, especially not in smaller vehicles. Active ballast systems often include tanks or other reservoirs connected with pumps or pistons used to drive fluid into and out of a reservoir internal of the vehicle, changing vehicle's displaced fluid volume and therefore buoyancy. Active ballast systems require additional control and electrical power, increasing the demands the ballast system puts on the vehicle. In large vehicles, such as full-sized submarines and surface ships, an active ballast system does not present a significant demand in terms of overall space, power consumption or complexity. However, in small submerged vehicles, especially small AUVs, an active ballast system will significantly impact component complexity and mission duration.
The need for proper ballasting and pitch optimization is exemplified in simpler vehicles having fewer appendages contributing to the overall drag. The appendage drag for one such vehicle, the Remote Environmental Monitoring UnitS (REMUS) AUV, with its various antennas and transducers, might be responsible for 75% or more of the total drag, translating to around 3-4% additional drag per degree of pitch due to the change in projected frontal area as pitch is increased. However, for a simple streamlined body, as might be expected from a small, low-power consuming, low-cost AUV, form drag can be upwards of 15% additional drag per degree of pitch. Pitch and positive buoyancy is discussed in more detail in Palmer et al. 2009 (Trans RINA, Vol 151, Part A3, International Journal of Maritime Engineering, 2009 July-September), incorporated by reference herein. Additionally, if the vehicle is ballasted to be neutrally buoyant, the energy savings from level flight (or flight with smaller pitch angles) could extend the duration a typical mission by nearly 50%. Furthermore, once neutrally buoyant, an AUV can go into a low-power drifting sleep mode, waking up periodically to communicate or take measurements, extending its endurance many times over. The current trends towards lower-power processors and sensors and the desire for longer AUV missions necessitate slower speeds and moreover some form of ballast system to unlock the full potential of these platforms.
In AUV design and operation, regardless of vehicle size, there is always a trade-off between speed and endurance. Slower speeds equate to longer missions because drag is proportional to the square of the velocity. When the power loads of various payload sensors are taken into consideration, an optimal speed emerges that corresponds to a maximum theoretical range. However, the pitch angles required to counteract positive buoyancy complicate this analysis because they contribute an additional drag term that is inversely proportional to speed. At higher speeds, this term is minimized because smaller pitch angles are required to balance the buoyant force. At lower speeds, however, the lift must be balanced using higher pitch angles, which effectively increases the cross-sectional area of the vehicle and subsequently the drag.
One solution to construct long-duration autonomous vehicles, are autonomous underwater gliders (AUGs). AUGs most often forgo a typical propeller propulsion system and instead use a pump to repeatedly change buoyancy, creating a propulsive force (termed a buoyancy engine). While AUGs use ambient pressure to their advantage, they do so for propulsion, and not as a driver of buoyancy change without an active, pumping component.
Another use of buoyancy for underwater craft is described in U.S. Pat. No. 3,204,596 by Fallon. Fallon's Hydroglider relates to a propellerless submarine that has a pressurizable compartment with one or more collapsible bags, each filled with a gas. The Hydroglider fits a single diver, and that person controls a pump to move ambient water into and out of the pressurized bags. The movement of the ambient water (and therefore a density and buoyancy change) acts much like modern AUGs, producing a forward motion. In addition to the human-powered glider, Fallon describes a buoyancy controlling mechanism of two pods, one above and one below the Hydroglider. The diver operates a bellows to drive ambient water into the pod below the craft, reducing the crafts buoyancy. The bellows allows the diver to over pressurize the lower pod, and give the craft lift by releasing the pressurized water. The upper pod can be connected to the lower pod and provides a stabilizing effect with no net change in buoyancy. Such a system fails to provide a truly passive (i.e. not diver activated) system that provides a changing vehicle buoyancy by means of the ambient water pressure.
A buoy containing a variable buoyancy portion without any rigid structure, such as a compressible bladder or compressible foam which changes shape to alter buoyancy, is disclosed in U.S. Pat. No. 9,272,756 by Thomson et al.
Therefore, alternative systems are needed to achieve neutral buoyancy, allowing operation at smaller pitch angles, without adding complexity or drag to the vehicle.